This invention relates to temperature sensors used for the accurate measurement of temperature, particularly in hostile environments. The application relates to the measurement of temperature with high resolution over a predetermined dynamic range.
The most commonly used temperature sensors of the prior art which provide an electrical signal output are thermocouples. Such devices are constructed by joining two pieces of different metals, often in the shape of thin wires, at a junction located in the environment whose temperatures are to be measured. Temperature changes at this junction modify the electrical potential of each of the metals, and, since the metals react differently, a potential difference is generated between the two metals at the junction. Such thermocouples provide a high degree of temperature sensitivity, providing as much as 60 millivolts of signal increase per degree celsius of temperature change. These thermocouples can be directly connected to analog or digital gauges to provide an accurate, repeatable measurement of temperature at the thermocouple junction.
Calibration tables exist for all common metal pairs, such as chromel-constantan or iron-constatan. Additionally, such thermocouples operate over broad temperature ranges from as low as -150 degrees Celsius to as high as 2500 degrees Celsius with relatively small error (sometimes as low as 0.5% over the entire temperature range). Because thermocouples have a low mass, and therefore a low thermal mass, they reach equilibrium with their environment very rapidly, especially when they are constructed to be extremely thin. Response times as low as 1 millisecond are known. While thermocouples can be protected to withstand hostile chemical environments, they are susceptible to induced errors from electromagnetic interference. In addition, unless well-protected by shielding which would necessarily increase the thermal mass and the response time of the thermocouple, these devices may be sensitive to some corrosive environments.